1. Field of the Invention
The present invention relates to a rear wheel steering system for a vehicle for steering the rear wheels together with the front wheels.
2. Description of Related Art
Japanese Patent Unexamined Publication (kokai) No. 152,977/1987 discloses a rear wheel steering system for a vehicle, in which its rear wheel steering mechanism is provided with a power steering unit which has a power cylinder for generating a power for steering the rear wheels in response to a liquid pressure, the power cylinder being connected through a high pressure passage to a pump as a source of generating the liquid pressure as well as through a low pressure passage to a reservoir tank. From the viewpoint of a fail safe, this system is provided with a spring for biasing the rear wheels toward the neutral position as well as with a fail valve which communicates the liquid chambers in the power cylinder with each other for placing the power cylinder in its neutral position. The pump for the power steering unit is always driven during operation of the engine, so that this pump tries to keep supplying the liquid pressure to the power cylinder even at the time of a fail.
In order to ensure a fail safe, demands have been made to release the liquid pressure in the power cylinder while keeping a supply of no liquid pressure to the power cylinder from the pump for the power steering unit. From this point of view, it is considered that a relief passage is separately disposed so as to communicate the high pressure passage with the low pressure passage and that a fail valve is disposed on the relief passage, the fail valve being made to open at the fail time.
It is further desired that the power cylinder is allowed to gradually release the liquid pressure as slow as possible in order to prevent the rear wheels from steering to the neutral position. At this time, the relief passage is provided with a restrictor. The position of this restrictor, however, may suffer from the disadvantage that the liquid pressure would not be released from the pump to the low pressure passage to a sufficient extent due to the action of the restrictor, thereby suppressing a release of the liquid pressure from the power cylinder.